In current 3GPP LTE (Third Generation Partnership Project Long Term Evolution) communication systems, that is, Releases 8, 9, and 10, downlink (DL) control signaling from an eNodeB is received by a user equipment (UE) in the first three (or four) symbols of a subframe (typically less than 3 for large system bandwidths such as 5 MHz, etc and less than 4 for smaller system bandwidths such as 1.25 MHz). The control channel duration is signaled on the Physical Control Format Indicator Channel (PCFICH) or sent via higher layer signaling. The remaining symbols are typically used for receiving user data, system information, synchronization signals, etc. For example, FIG. 1 depicts an exemplary subframe structure of the prior art. Control signaling is spread across an entire carrier bandwidth (for example, 10 Megahertz (MHz)) of the first three symbols of the subframe and is received by the UE on a Physical Downlink Control Channel (PDCCH). User data is received by the UE on the Physical Downlink Shared Channel (PDSCH), and in select Resource Blocks (RBs) of the PDSCH occupying either in the entire carrier bandwidth or a portion of it. In the Rel-8 LTE and beyond LTE systems such as Rel-10 (also known as LTE-Advanced), the base station transmits using an OFDM modulation scheme on the downlink and the UEs transmit on the uplink using a single carrier frequency division multiple access (SC-FDMA) scheme and/or Discrete Fourier Transform Spread OFDM (DFT-SOFDM). In a Frequency Division Duplex (FDD) operation, the frame structure in the uplink and downlink, each comprises a 10 millisecond (ms) Radio frame, which in turn is divided into ten subframes, each of 1 ms duration, wherein each subframe is divided into two slots of 0.5 ms each, wherein each slot contains a number of OFDM symbols. In Time-Division Duplex (TDD), the Radio Frame is still divided into 10 subframes, but the subframes can be of different types—downlink subframes, uplink subframes, and special subframes that have a downlink sub-portion (or region, DwPTS), guard sub-portion (or period or GP) and an uplink sub-portion (UpPTS). The DL subframes are typically of two types—regular DL subframes that contain CRS in both slots and Multicast-Broadcast Single-Frequency Network (MBSFN) subframes that contain CRS only in the beginning portion of the subframe while the rest of the subframe contains no CRS. The UEs receive downlink control information (DCI) in the control region. There are various DCI Format types for carrying a variety of control information. For example, the DCI Format 0 is used to schedule uplink transmissions and typically comprises scheduling information fields such as a modulation and coding scheme (MCS) index, Resource block allocation, Hopping flag, New Data Indicator, Transmit power control (TPC) command, and/or hybrid ARQ information. The user identification or user ID (UEID) is typically embedded within the CRC bits (e.g. the CRC is scrambled based on UEID). The DCI Format 1A is a compact scheduling grant used to schedule a single transport block and includes fields similar to those in DCI Format 0, and additional fields such as Redundancy Version (RV). DCI Format 2A is used to schedule two transport blocks in the downlink using open-loop MIMO whereas DCI Format 2B is used to schedule two transport blocks in the DL using closed-loop MIMO and CRS. DCI Format 2C is used for scheduling DL transmissions in transmission mode 9, where the up to two transport blocks may be scheduled using DMRS.
In order to decode the information sent on PDCCH, the UE needs to perform channel estimation for coherent demodulation of the PDCCH. To perform channel estimation, the UE receives Reference Signals (RSs), for example, pilot symbols, that are Cell-specific reference signals (CRS) and included in the subframe and that are associated with one or more antenna ports. For example, in 3GPP LTE Releases 8, 9, and 10, the UE uses the CRSs associated with one or more of antenna ports 0, 1, 2, and 3 for receiving the PDCCH. The number of antenna ports used for demodulating control channels is determined from decoding of the Physical Broadcast Channel (PBCH) that is transmitted in known Resource blocks in subframe 0. Typically, transmit diversity scheme is used when more than one antenna port is used for control channel demodulation. An antenna port is defined such that a channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. The Reference Signals (RS) structure for antenna ports 0, 1, 2, and 3 is shown in FIG. 1, wherein RSs labeled R0 are resource elements carrying RSs associated with antenna port 0, RSs labeled R1 are resource elements carrying RSs associated with antenna port 1, RSs labeled R2 are resource elements carrying RSs associated with antenna port 2, and RSs labeled R3 are resource elements carrying RSs associated with antenna port 3.
For 3GPP LTE Release 10, in order to demodulate user data (sent on PDSCH), the UE can either use the RSs associated with antenna ports 0, 1, 2, and 3 or the UE can use RSs associated with other antenna ports, such as antenna ports 7, 8, 9, 10, 11, 12, 13, and 14, that is, the UE can use RSs associated with all or a subset of these antenna ports, based on the transmission scheme used for PDSCH reception. The RSs associated with these other antenna ports are typically referred to as “UE specific reference signals (UERSs)” or “Demodulation reference signals (DMRSs) or “Dedicated reference signals (DRS).” The RSs associated with antenna ports 0, 1, 2, and 3 are typically referred to as “Common Reference Signals (CRSs).” In transmission schemes based on CRS, the UE may use one or more of antenna ports 0,1,2,3 and for transmission schemes based on DMRS, the UE may use one or more of antenna ports 7,8,9,10,11,12,13,14. The actual number of spatial transmission layers and the associated antenna ports when using DMRS to decode PDSCH may be determined by the UE based on the downlink control channel (DCI) information associated with PDSCH. Typically, both CRS and DMRS are not simultaneously used to demodulate data in PDSCH. While the CRSs are sent across the entire carrier bandwidth by the eNodB, DMRSs can only be present in those RBs for which the UE has a PDSCH assignment. Therefore, when receiving PDSCH using DMRS, the UE can only use the DMRS present on those RBs for which it has a PDSCH assignment.
For 3GPP LTE Release 11 (the next generation LTE system), it is envisioned that new DL control signaling will be sent by the eNodeB to the UE in symbols that span a first time slot of the subframe or in symbols that span both the first and a second time slot of the subframe. The new DL control signaling is generally referred to as Enhanced-PDCCH (EPDCCH). Unlike the PDCCH, which is transmitted across the entire channel bandwidth, a UE is expected to receive the EPDCCH in a set of RBs that may span only a portion of the carrier bandwidth in frequency domain. Also, unlike the PDCCH, which is received by the UE using CRS, it is envisioned that the EPDCCH can be received by the UE using DMRS.
The new DL control signaling, that is, the EPDCCH, is expected to be used to complement the downlink control channels, that is, the PDCCH, of the existing 3GPP LTE Releases 8/9/10 for supporting features of Long Term Evolution-Advanced (LTE-A) Release 11+, such as CoMP (Coordinated Multi-point Transmissions) and enhanced Multiple-Input Multiple-Output (MIMO) techniques, including Multi-User MIMO (MU-MIMO). Such control channel enhancements may allow beamformed frequency-selective control transmission, for example, using dedicated control transmission to a UE via use of DMRSs and allocation of spatially multiplexed control channels to a single user MIMO (SU-MIMO) and/or to MU-MIMO control channels. Typically, such new control channels may be defined as Frequency Division Multiplexed (FDM) control channels that occupy fewer downlink (DL) RBs compared to the total number of DL RBs. Another new DL control channel, that is, a Relay Physical Downlink Control Channel (R-PDCCH), carries downlink control information (DCI) for Relay Nodes (RNs). The R-PDCCH has a mode of operation (no cross-interleaving, DMRS-based) wherein the DCI conveyed to an RN occupies a small number of RBs (typically 1, 2, 4, or 8 RBs) and the set of RBs configured for control channel transmission is signaled via the Radio Resource Control (RRC) protocol. Additionally, the RN assumes a fixed antenna port, that is, antenna port 7 (AP7), and a fixed scrambling identifier (ID), that is, scrambling ID 0, for receiving the DCI (that is, there is no MU-MIMO). DL grants are sent in a first time slot and uplink (UL) grants are sent in a second time slot.
Given a fixed or limited blind decoding budget per user, such as a UE or an RN, there is a need to develop an efficient control channel design that also addresses control channel blocking problem in a multi-user control scenario. The 3GPP standards do not address how a control channel search space and a blind decoding for a user is configured with respect to antenna ports, etc. For example, if all of the users in a cell are configured within a same set of RBs for control and same antenna port (like the R-PDCCH), then the control blocking rate would be high as all users would be trying to occupy the same resource. On the other hand, if all users in a cell are configured with different sets of RBs different antenna ports for DL control signaling, then there is resource wastage.
Therefore, a need exists for defining a control channel search space and/or a blind decoding configuration that can improve resource efficiency, multiplexing efficiency, and reduced and randomized control channel blocking, while also allowing a reasonable blind decoding complexity at the UE.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.